The goal of this project is to determine mechanisms that regulate regenerative and neurodegenerative responses in dentate gyms granule cells following axotomy of the principal glutamatergic input, the perforant path (PP). In this model, a precise lesion is performed in adult mice that transect the PP without damaging the hippocampal formation, allowing for a high-resolution assessment of lesion-induced synaptic plasticity (LISP). The hypothesis being tested is that a sublethal, excitotoxic mechanism occurs following PP transections leading to short- and long-term transneuronal reorganization of granule cells and granule cell dendrites. To attain the necessary level of cellular and subcellular resolution, a "molecular fingerprint" of dentate gyms granule cells as well as granule cell dendrites is performed. This is done using a single cell amplified antisense (aRNA) amplification methodology combined with cDNA array technology to provide an extensive, concurrent representation of hundreds of genes (approximately 220 cDNAs on custom-designed arrays and 6500 cDNAs on high-density cDNA microarrays), with emphasis on detecting alterations in glutamate receptor (GIuR) gene expression. Thus, the regulation of mRNAs for GluRs and other transcripts of glutamatneric neurotransmission is used as a biological marker to differentiate plastic sprouting responses from neurodegenerative changes that occurs across the time course of the lesion. The excitotoxic hypothesis is challenged by examining granule cell expression profiles following the delivery of excitatory amino acid antagonists prior to PP transections. Further, a molecular fingerprint of excitotoxicity in granule cells is performed by delivery of kainate for direct comparison to PP transections. This application applies a broad scale functional genomics approach for determining the molecular substrates underlying LISP, and tests the excitotoxic hypothesis following PP transections. Alterations in GluRs and other relevant transcripts that help to determine cellular sequelae following LISP are relevant to understanding the cellular and molecular underpinnings of activity-dependent responses within hippocampal circuits and are also directly relevant to uncovering the mechanism(s) underlying synaptic and neurodegenerative changes in the brains of humans with a variety of neurodegenerative disorders. Thus, this novel cellular and molecular paradigm shift of the well-characterized PP transection model in vivo may help to identify specific transcripts, and ultimately, proteins that are responsible for transneuronal degeneration and dendritic remodeling following axotomy.